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Abstract: Catalytic decomposition of hydrocarbons is a major contributor to carbon formation during hydrocarbon reforming. This work investigates the effects of temperature and hydrocarbon decomposition intermediates on carbon deposition during catalytic isooctane decomposition over ceria–zirconia supported nickel. The carbon deposition rate and hydrocarbon concentrations were monitored in real-time via thermogravimetry and FT-IR spectroscopy. Carbon-deposited catalysts were characterized by temperature-programmed oxidation (TPO) and scanning electron microscopy (SEM). Carbon deposition rate, morphology, and quantity were strongly dependent on decomposition temperature (T); three distinct decomposition temperature regimes were identified, i.e. T ≤575°C, 575°C< T <725°C, and T ≥725°C. Gaussian deconvolution of TPO data and carefully designed oxidation experiments indicated the presence of different types of carbon: coating, filamentous, and pyrolytic carbon. Catalytic and homogeneous decomposition of isooctane, propene, and methane suggest that smaller hydrocarbon fragments, typically formed during reforming of large hydrocarbon molecules, play an important role in carbon deposition. Specifically, low-temperature decomposition of isooctane into olefins, such as propene or isobutene, leads to the formation of both coating and filamentous carbon deposits. At high temperatures, decomposition of acetylene and methane leads to the formation of significant quantities of coating and pyrolytic carbon deposits in addition to filamentous deposits. [ABSTRACT FROM AUTHOR]

Abstract: The extent and nature of carbon deposition that occurs during catalytic reforming reactions of hydrocarbons depends strongly on the reaction conditions and the surface properties of the catalyst. In conventional packed bed reactors it is difficult to track the spatial distribution of carbon and link it to the location of individual catalyst particles. The present work uses monolith-based nickel catalysts to investigate the axial carbon gradient post n-dodecane reforming. The carbon, which was deposited during autothermal reforming (ATR), partial oxidation (POX), and steam reforming (SR) over Ni/cerium-zirconium oxide (CZO)/monolith and Ni/monolith catalysts, was characterized by a combination of temperature-programmed oxidation (TPO), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The amount, location, and type of carbon deposited strongly depended on the type of reforming reaction. The majority of carbon deposition was concentrated inside cordierite pores of the monolith. In general, the catalysts showed the expected trend of increased filamentous carbon deposition down the length of the monolith, reflecting the axial variation in temperature and oxygen concentration, except for POX over Ni/monolith catalyst where massive coating carbon deposition occurred in the upstream section of the monolith. [Copyright &y& Elsevier]

Abstract: The carbon deposition behavior of Ni-based monolith reforming catalysts was studied during n-dodecane autothermal reforming, partial oxidation, and steam reforming. One catalyst formulation was nickel deposited on cerium zirconium oxide (CZO) coated monolith, while the second formulation was nickel directly deposited on bare monolith. In both formulations, a series of catalysts with a range of nickel loadings (0–16wt.%) were prepared to examine the influence of nickel loading on carbon deposition and to elucidate the benefits of the reducible oxide support CZO on carbon deposition. Carbon deposition was generally more pronounced at higher nickel loadings and on catalysts lacking CZO. Nickel supported on bare monolith suffered from excessive carbon deposition and carbon-induced monolith disintegration. The morphologies of carbon were determined by scanning electron microscopy (SEM). Temperature programmed oxidation (TPO) indicated the presence of two types of carbon. The low-temperature TPO peak can be attributed to coating carbon, while the high-temperature peak corresponds to filamentous carbon structures, including large whiskers and smaller filaments. Accumulation of whisker carbon had a deleterious effect on the monolith substrate resulting in the physical destruction of some samples. X-ray diffraction (XRD) gave no evidence for the presence of graphite or carbide species in carbon-deposited catalysts. The experimental results of this study are used to diagnose the causes for nickel catalyst deactivation during autothermal reforming and for proposing strategies to mitigate the deactivation. [Copyright &y& Elsevier]

Abstract: On-board reforming of liquid fuels is attractive for fuel-cell-powered auxiliary power unit (APU) applications in heavy-duty vehicles. However, a technology barrier is catalyst deactivation due to carbon deposition. In this work, we studied carbon identification and the effect of nickel loading on carbon growth during isooctane autothermal reforming (iC8-ATR) over Ni-supported Ceph name="sbnd" />O (Ni/CZO) catalysts. The crystallographic phase, type, deposition amount, and morphologies of deposited carbon were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Two types of carbon, coating and filamentous, were identified by SEM, with binding energies of 281.6 and 284.5 eV in C1s XPS spectra and oxidation temperatures of 392 and 572 °C in TGA curves, respectively. Both coating and filamentous carbon were X-ray amorphous for nickel loadings ⩽5%, even though deposited carbon reached up to 60.2%. Over the entire nickel loading range of 1–15%, with corresponding nickel crystal sizes of 30–103 nm, the coating carbon formation rates were low (0.04 g/m2 Ni h) and independent of nickel particle size. The growth rates of filamentous carbon increased with increasing nickel particle size. Filamentous carbon growth may require not only a critical nickel particle size, but also a certain minimum number of nickel particles per unit support area. The results suggest a strategy for inhibiting the growth of filamentous carbon on Ni/CZO catalysts during autothermal reforming of liquid fuels. [Copyright &y& Elsevier]

Abstract: Autothermal reforming (ATR) of n-dodecane was systematically investigated with O/C=0.6 and H2O/C=2.0 over nickel-based catalysts supported on cordierite monoliths. Both nickel supported on monolith (Ni/monolith) and nickel supported on cerium–zirconium oxide (CZO) loaded on monolith (Ni/CZO/monolith) were tested for ATR activity. The influence of nickel weight loading (0–16 wt%) on ATR product yields was examined. Experimental results showed that 2 wt% Ni/CZO/monolith was an optimal composition for ATR. The roles of Ni and CZO were determined by comparing n-dodecane conversion, oxygen conversion, the extent of reforming, and product yields (i.e., CO, CO2, and H2). The reaction studies indicated that nickel catalyzed the conversion of n-dodecane by POX and SR, yielding a hydrogen rich effluent, whereas CZO alone catalyzed the conversion of n-dodecane though catalytic oxidation and cracking, yielding an effluent rich in smaller hydrocarbon species. Nickel supported on CZO showed the greatest hydrogen, carbon monoxide, and carbon dioxide yields of the catalysts studied. ATR, partial oxidation (POX), steam reforming (SR), and the influence of reaction temperature on ATR were studied separately and compared at similar conditions over 2 wt% Ni/monolith and 2 wt% Ni/CZO/monolith to elucidate the primary reforming reactions. An n-dodecane ATR reaction schematic is postulated, and the influences of homogeneous activity and oxygen conversion are discussed. It is proposed that POX, homogeneous cracking, and oxidative cracking are the major routes for n-dodecane conversion during ATR, whereas POX of n-dodecane and SR of smaller hydrocarbons are the primary routes to reforming products (i.e., H2, CO, and CO2). [Copyright &y& Elsevier]

Abstract: The performance of a Ni/Ce0.75Zr0.25O2 catalyst for the autothermal reforming of isooctane is evaluated and compared using typical packed bed reactors, steel microreactors, and quartz microreactors. Microchannel reactor systems have in recent years been proposed as attractive alternatives to traditional catalytic packed bed reactors due to their improved heat and mass transfer properties. The microreactors employed in this study are comprised of stackable layers, with FeCrAlloy metal foams serving as catalyst substrate. To deploy the Ni/Ce0.75Zr0.25O2 catalyst in the microreactors, the metal foam substrates were coated with a catalyst washcoat formulation. Several factors are discussed that may influence the microreactor performance, with emphasis on the different contributions of heat transfer mechanisms between the active reaction zone and its surroundings, the materials of construction, and reactor aspect ratio. Heat effects were found to be particularly important in determining performance for the steel microreactors. [Copyright &y& Elsevier]

Highlights: [•] Cerium doping affects the crystallography of anatase. [•] Ni crystallites size decreases with Ce doping. [•] The insertion of Ce into the lattice of anatase creates Tiα sites. [•] Incorporation of Ce improves the catalytic performance for “CO+NO” reaction. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes to what extent Ni particle size affects the sulfur-tolerance of ceria-zirconia supported Ni catalysts during autothermal reforming (ATR) of isooctane. Particle size was isolated as an experimental variable by preparing catalysts with a range of Ni loadings that had nearly identical Ni surface areas. Under sulfur-free conditions, isooctane conversion and synthesis gas yield increased as the Ni particle size increased, contrary to the expectation that smaller particle sizes with lower Ni coordination would be more active. However, larger Ni particles proved to be more vulnerable to sulfur poisoning. The poor ATR activity of small Ni particles can be attributed either to a lack of sufficiently large nickel surface ensembles, or to their higher propensity to form nickel oxides under reaction conditions. This contribution suggests that, under typical ATR conditions, more highly dispersed Ni catalysts will not result in elevated sulfur tolerance. [Copyright &y& Elsevier]

Abstract: Catalytic reforming of liquid hydrocarbon fuels is challenging due to potential deactivation via carbon deposition and sulfur poisoning. To gain a better understanding of the effect of sulfur on the deactivation of Ni/Ce0.75Zr0.25O2 catalysts, isooctane conversion to syngas was studied in the presence of small amounts of thiophene under various O/C and H2O/C ratios representing steam reforming, partial oxidation, and autothermal reforming conditions. It was found that depending on the reaction conditions, thiophene underwent different degrees of desulfurization, leading to the formation of H2S. Under reaction conditions leading to nearly complete conversion of thiophene to H2S, the nickel catalyst lost only a small amount of its initial activity, but then maintained stable performance over longer times on stream. In contrast, reaction conditions under which thiophene emerged unconverted from the reactor led to severe and continued deactivation of the catalysts. Furthermore, co-feeding thiophene with isooctane caused significant increases in the temperature profile of the reactor and an increased amount of olefins were seen as products of the reaction, indicating that sulfur deactivated primarily catalyst sites responsible for endothermic steam reforming reactions, while having less impact on exothermic partial oxidation reactions. Controlling the reaction conditions in such a way as to generate sufficient hydrogen concentrations in the catalyst bed for effectively desulfurizing thiophene to H2S appears to be the key to maintain stable catalytic activities in the presence of sulfur compounds. [Copyright &y& Elsevier]